The present invention relates generally to circuits for interfacing telephone equipment to a four wire telephone line and, in particular, to a circuit which supplies a DC bias and optional sidetone to a telephone headset and in addition provides signal conditioning between the headset and the four wire line.
In present day telephone systems, it is often desirable for an operator using a telephone headset to be able to connect directly into a four wire telephone line. However, certain problems exist which may form a danger to the operator as well as cause poor operation of the headset. For example, signal levels on a telephone line can range from +16.0 dbm to -7.0 dbm. Furthermore, it is possible that large signal levels can momentarily occur on the telephone line. Ear damage to an operator may occur when signal levels on the telephone line are greater than -15 dbv, which may be applied to the headset. A -22 dbv tone is considered to be annoying. Normal speech levels are at approximately -41 dbv for good operation of the headset. Therefore, proper signal conditioning is needed for acceptable operation of the headset, as well as, providing for the safety of the operator.
In addition, it is desirable that a predetermined bias voltage be available for the microphone in the headset. In terms of cost of manufacturing there is a need to provide the DC biasing voltage with a minimum of circuitry.
The present invention provides solutions to these problems in the prior art.